Using Morphological Distance to Determine Genera

Depending on the taxonomist (and, for early forms, the phylogeny), the human total group includes anywhere from one genus (Homo) to ten (Ardipithecus, Australopithecus, Homo, Kenyanthropus, Orrorin, Paranthropus, Paraustralopithecus, Praeanthropus, Sahelanthropus, and Zinjanthropus— not even mentioning obsolete ones like Telanthropus and Pithecanthropus). We could try to clean up this mess by creating phylogenetic definitions and reinterpreting the genera as clades, except that all of the type species are thought by at least some researchers to be ancestral forms (with the exceptions of Homo sapiens,Paranthropus robustus, and Zinjanthropus boisei). For example, if Australopithecus is a clade that includes Australopithecus africanus, then it might also include Homo, and genera are not allowed to overlap under the ICZN.

Ardipithecus anamensis (Not in Praeanthropus, despite sometimes being synonymized with afarensis! Although it should be noted that ramidus is much better known now than in 2004, so this may have changed.)

Ardipithecus ramidus

Australopithecus

Australopithecus africanus

Homo

Homo ergaster

Homo habilis (although it is closer to garhi than to sapiens!)

Homo sapiens

Kenyanthropus

Kenyanthropus garhi (!)

Kenyanthropus platyops

Kenyanthropus rudolfensis (although it is closer to ergaster and habilis than to platyops, it is still closer to platyops than to sapiens)

Orrorin

Orrorin tugenensis (not included in the study, but this is nomenclaturally where it would go unless found to be a synonym)

Paranthropus

Paranthropus robustus

Paraustralopithecus

Paraustralopithecus aethiopicus

Praeanthropus

Praeanthropus afarensis

Sahelanthropus

Sahelanthropus tchadensis

Zinjanthropus

Zinjanthropus boisei

Not included: species that are not types and were not included in the study, like Ardipithecus kadabba (scrappy craniodental remains), Australopithecus sediba (hadn't been discovered in 2004), Homo heidelbergensis (pretty close to sapiens anyway), etc.

It must be said that Zinjanthropus and Paraustralopithecus are not that commonly used. If we remove Paraustralopithecus, then aethiopicus predictably falls into Zinjanthropus. If we remove Zinjanthropus as well, then both aethiopicus and boisei predictably fall into Paranthropus.

Praeanthropus is also not that widely used. If we remove that as well, we get:

Kenyanthropus is a rather controversial genus. If we remove it, we get:

Ardipithecus

Ardipithecus anamensis

Ardipithecus garhi

Ardipithecus platyops

Ardipithecus ramidus

Australopithecus

Australopithecus afarensis

Australopithecus africanus

Australopithecus rudolfensis (still refuses to go with sapiens!)

Homo

Homo ergaster

Homo habilis

Homo sapiens

Orrorin

Orrorin tugenensis

Paranthropus

Paranthropus aethiopicus

Paranthropus boisei

Paranthropus robustus

Sahelanthropus

Sahelanthropus tchadensis

If we also remove Sahelanthropus, then tchadensis goes easily into Ardipithecus. I assume tugenensis would as well, if we removed Orrorin, but that wasn't included in the study since the craniodental material is so scant. If we remove Paranthropus, its species go very, very reluctantly into Australopithecus (by which I mean despite not being that close to africanus):

Gorilla aethiopicus (!!! although gorilla only beats sapiens by a hair)

Gorilla beringei

Gorilla garhi (!!!)

Gorilla gorilla

Gorilla tchadensis (admittedly, this was suggested by Senut and Pickford)

Homo

Homo africanus

Homo boisei

Homo ergaster

Homo habilis

Homo platyops

Homo robustus

Homo rudolfensis

Pan

Pan afarensis (!!)

Pan anamensis(!!)

Pan paniscus

Pan ramidus (!!)

Pan troglodytes

incertae sedis

Orrorin tugenensis

Yes, some of the stem-humans get pulled in with chimpanzees or gorillas! And there seems to be little rhyme or reason as to which go where. The splitting of the "robust australopithecines" is most bizarre (although, as noted, it only takes a tiny nudge to put aethiopicus in Homo with the others).

In summary:

This is just one matrix, only focusing on one area of the anatomy.

There are huge amounts of uncertainty with some of these taxa.

Even without the uncertainty, there is no objective way to measure morphological distance.

Even if there were, it might not be a good idea to use it to determine generic boundaries.